1. Technical Field
The present invention relates to a photographic zenith telescope which detects the light of interest coming from the zenith and which uses its movement to perform astronomical observation of the behavior of a particular celestial body.
2. Related Art
FIG. 6 shows the structure of a conventional photographic zenith telescope. The telescope condenses the rays of light from a celestial body of known position either at or near the zenith and a slight positional offset of the image on a detector is observed for the typical purpose of measuring a deflection in the revolution of the earth on its own axis. In FIG. 6, reference numeral 1 designates an objective lens for condensing the rays of light from an object of interest, 2 is mercury, 3 is a vessel of mercury 2, 4 is a detector, and 5 designates structural members for supporting the objective lens.
The rays of light from a celestial body to be observed are condensed by the objective lens 1, reflected by a reflecting surface created by the surface of the mercury 2, and imaged by the detector 4 positioned at the center of the objective lens 1. In this optical system, the objective lens 1 performs a single plane reflection to condense the incident light, so the distance between the objective lens 1 and the surface of the mercury 2 is about one half the focal length of the objective lens 1. The photographic zenith telescope having the structure shown in FIG. 6 will tilt for several reasons such as thermal deformation of the structural members 5. If this occurs, the objective lens 1, mercury vessel 3 and detector 4 also tilt en masse but the surface of the mercury 2 maintains the same plane, so the rays of light from the celestial body being observed form a focused image at the same point on the detector 4. If the revolving axis of the earth tilts due to small deflection, the direction of gravity that determines the surface of the mercury 2 also tilts and the rays of light from the celestial body of interest form an image in an offset position on the detector 4. This is the principle for measuring a deflection in the revolution of the earth on its own axis.
In the conventional photographic zenith telescope described above, the mercury 2 has to be positioned away from the objective lens by a distance about one half its focal length and this inevitably increases the size of the optical system in the direction of the optical axis. The mercury 2 in the vessel 3 is used as a plane reflector mirror but because of its high density, the optical system is undesirably heavy. As a further problem, the mercury vessel 3 has a temperature distribution in the mirror surface and if the mercury 2 expands, its vertical size changes to produce asperities in its mirror surface, producing a blurred or offset image on the detector 4.
The present invention is accomplished under these circumstances and its primary objective is to provide a photographic zenith telescope that has a smaller and lighter optical system and which is less prone to produce a blurred or offset image on the detector.
The stated object of the invention can be attained by the photographic zenith telescope recited in aspect 1 which comprises an objective lens for condensing the rays of light under observation, a plane reflector mirror that is in a face-to-face relationship with the objective lens and placed in a position distant from the objective lens by about one half its focal length for reflecting the incident light from the objective lens, a detector positioned at the center of the objective lens through which the optical axis passes, the detector coinciding with the focal point of the reflected light from the plane reflector mirror, and a reflector mirror floating vessel containing a fluid on which the plane reflector mirror floats.
The photographic zenith telescope according to aspect 1 may be adapted as recited in aspect 2, wherein the reflector mirror floating vessel contains mercury on which the plane reflector mirror floats.
The photographic zenith telescope according to aspect 1 may also be adapted as recited in aspect 3, wherein the plane reflector mirror is equipped with intervening pads that cause the plane reflector mirror to float in the reflector mirror floating vessel.
The stated object of the invention can also be attained by the photographic zenith telescope recited in aspect 4 which comprises an objective lens for condensing the rays of light under observation, a plane main mirror that is in a face-to-face relationship with the objective lens and placed in a position distant from the objective lens by about a quarter of its focal length for reflecting the incident light from the objective lens, a plane auxiliary mirror that is in a face-to-face relationship with the plane main mirror and positioned between the objective lens and the plane main mirror for causing the reflected light from the plane main mirror to be reflected back to the plane main mirror, and a detector positioned at the center of the objective lens through which the optical axis passes, the detector coinciding with the focal point of the reflected light from the plane main mirror.
The photographic zenith telescope according to aspect 4 may be adapted as recited in aspect 5, wherein the plane reflector mirror and the plane auxiliary mirror float in a floating vessel.
The photographic zenith telescope according to aspect 1 or 4 may also be adapted as recited in aspect 6, wherein the detector is positioned at the principal point of the objective lens.
The photographic zenith telescope according to aspect 1 may also be adapted as recited in aspect 7, wherein the reflector mirror floating vessel has a plurality of fluid reservoirs, a sealing membrane for sealing the fluid contained in the fluid reservoirs, and a bypass tube interconnecting the spaces in the plurality of fluid reservoirs.